None.
1. Field of the Invention
The present invention generally relates to a driver circuit for a variety of symmetric load systems. The invention specifically describes a power circuit based on a balanced capacitive loading method wherein the load itself acts as an energy storage element in the energy balance system. In preferred embodiments, the circuit redirects power from a near lossless load to either another load of the same type or to a capacitively equal inactive element. At any given time, a portion of the symmetric load system acts as an actuator and the remaining pure capacitive portion functions similar to a power bypass capacitor.
2. Related Arts
A large class of active control devices incorporate small, high-force transductive mechanisms to develop mechanical force. Electrostriction mechanisms develop mechanical force by the interaction of electric fields within the transducer. Magnetostriction mechanisms develop mechanical force by the interaction of magnetic fields within the transducer. Transductive mechanisms are inherently lossless, therefore the energy pumped into the device is returned except for a small portion expended producing mechanical work.
Various power circuits are known within the art to drive transductive mechanisms. Linear driver circuits are the most common approach. Linear drivers are very inefficient in that return energy from the transductive mechanism is dissipated thermally and thereby no longer available to drive the mechanism. Some improved performance is obtained with class D implementations of the electronics, however, the issue of how to store the transient return energy remains unresolved.
A more attractive solution to reverse energy flow is a regenerative driver circuit as disclosed in U.S. Pat. No. 6,001,345 issued to Murray et al. on Jan. 4, 2000. However, the invention by Murray suffers two fundamental problems. First, the invention requires a negative impedance inverter that is both quite complex to achieve and never adequately demonstrated in practice. Secondly, the invention requires a large output bypass capacitor. The capacitor value is chosen according to
RLoadCFilter greater than  greater than 1/F
where F is the ripple frequency. The ripple current is in this case impressed by the transients in and out during switching. This leads to a minimal requirement of the output bypass capacitor, where
Cfilter greater than  greater than Cload
as to achieve a xcfx893db bypass. Consequently, the power bypass capacitor quickly becomes the dominating factor in terms of mass, volume, and performance at larger loads. The result is diminished advantages in terms of efficient power handling and compact implementation of the switching section in the drive topology.
The deficiencies of Murray et al. are best represented by example. Two symmetric load devices each consisting of two transductive elements are described in FIGS. 8 and 10. Two symmetric load devices each consisting of four transductive elements are described in FIGS. 11 and 12. For discussion purposes, piezo-mechanical motion is induced by piezoelectric transducers with 5.5 xcexcF capacitance operating at 200 volts peak amplitude. The total resultant piezo-capacitance is 22 xcexcF (4 times 5.5) for dually-opposed implementations and 44 xcexcF (8 times 5.5) for quadrature-opposed implementations. Circulating currents from the large piezo-capacitive load has a deleterious effect on a high-voltage power supply because of high ripple causing xe2x80x98gainxe2x80x99 ripple induced at the switching amplifier output. The result is decreased stability at higher loads. A bypass capacitor one-hundred times the piezo-capacitance, 2,200 xcexcF and 4,400 xcexcF respectively for the examples above, is required to lower the return ripple of the recirculating current. Such bypass capacitors must operate satisfactorily within the bandwidth of the system, at a minimum several hundred hertz. Such operating conditions provide serious design challenges for both regenerative and conventional driver circuits.
Conventional power circuits are designed to drive only one side of a transductive system. When applied to a symmetrically coupled transductive system, the lossless nature of the transducers requires nearly all of the input energy returned and either transferred out the system as thermal energy or recovered and redirected. If recovered, the energy is typically recycled with additional input side energy to drive the other symmetric load at the output side of the circuitry. The recovery-recycle methodology as applied to symmetrically coupled systems by conventional circuits produces large peaks in the power supply ripple current. Consequently, such systems are inherently unstable.
What is required is a power control circuit capable of rapidly redirecting energy between loads in a symmetrically coupled arrangement and specifically a system wherein said loads are transductive elements. The circuit should substantially reduce peak power loading without increasing total power demand. The circuit should eliminate the large bypass capacitor required in the related arts, thereby facilitating a smaller, lighter package. The circuit should eliminate the power supply related stability problems inherent to regenerative and conventional electronics.
A first object of the present invention is to provide a small, lightweight electronic driver circuit eliminating the need for large d.c. power bypass capacitors to drive transducer actuated symmetric reactive load systems.
A second object of the present invention is to provide more volumetrically efficient d.c. power section by effectively removing peak power requirements, leaving only low-level average power to be serviced.
A third object of the present invention is to provide regenerative efficiency without the need for a large d.c. power bypass capacitor to drive transducer-actuated systems.
A fourth object of the present invention is to provide increased stability when electrically powering transducer-actuated symmetric reactive load systems at higher charge levels.
To these ends, the present invention provides a regenerative class D power circuit attached to a symmetrically terminated reactive load system. The power circuit incorporates a new balanced capacitive loading method using the pure reactive portion of the load itself as an energy storage element in the energy balance system. In the present invention either a half-bridge FET or dual half-bridge FET switching topology controls charge-discharge between the two halves of a symmetric reactive load system. The invention can be implemented in the preferred embodiment consisting of a single half bridge or second embodiment consisting of dual half bridges driven 180 degrees out of phase. The topology of the present invention causes energy to be cycled from one side of the symmetric output load to the other side of the symmetric output load. Half-bridge averaging in the invention is externally commanded via a control module. When half-bridge averaging is commanded, an imbalance is caused producing current to flow in one desired direction only. The invention causes the charge to equilibrate between the two symmetric output loads in reference to the new average control module charge. The load on the driver at any given instant is the total output load, while load on the d.c. power supply is only the real power to the load used plus any switching losses. A control module, one example being a PWM, is employed as to institute power flow between symmetric loads as seen on the output side of the circuit. The present invention optimizes the coupling of energy in the L/C circuit comprising the symmetric loads as seen at the output of the circuitry.
The present invention minimizes power supply conditioning bypass capacitor requirements. Conventional half-bridge power supply circuits require a large bypass capacitor to filter all of the ripple current related to driving the reactive load. FIG. 1a shows a conventional half-bridge arrangement wherein the ripple is related to
(XLoad+Zload)xc3x97ILoad
In the present invention, the circuit is required only to filter the ripple current related to the real power dissipated in driving the compound symmetric, reactive or more specifically capacitive, load. In the present invention, the load is a priori symmetrically divided and this fact is used to terminate the circuit uniquely as shown in FIG. 1b. Thus, the current ripple is only related to
XLoadxc3x97XLoad
The present invention offers several key advantages over class C, class D and class D regenerative circuitry. The present invention is lighter and smaller with increased efficiency over the related arts. The present invention significantly reduces the high-voltage power supply bypass capacitor representing the largest component in class D and regenerative class D circuitry. The present invention enables larger effective output filter values in a smaller package thereby increasing robustness. Thus, the present invention enables the compact, lightweight implementation for driving high-voltage symmetric output load systems. The present invention effectively enables higher switching voltage into symmetric output reactive load systems thereby retaining the high efficiency of regenerative drivers.
The present invention is applicable to a wide range of transductive systems including bimorph mechanisms, inchworm devices examples of which are described in U.S. Pat. Nos. 3,902,084, 3,902,085, 4,874,979, and 5,751,090, quadrature MEMS (micro-electro-mechanical systems) gearing, piezoelectric powered scroll compressors an example described in U.S. Pat. No. 4,950,135, and piezoelectric activated optical communication devices. The advantage of the present invention is that it substantially reduces the instantaneous loads on the high-voltage power supply. This in turn, significantly reduces the power supply mass and volume. In contrast to power switching electronics in the related arts, the present invention is easily miniaturized due to the elimination of large power filter components.